class Solution:
    def findDisappearedNumbers(self, nums):
        import collections
        """
        :type nums: List[int]
        :rtype: List[int]
        """
        n = len(nums)
        res = []
        counter = collections.Counter(nums)
        for i in range(1, n + 1):
            if i not in counter:
                res.append(i)
        
        return res

s = Solution()
print(s.findDisappearedNumbers([4,3,2,7,8,2,3,1]))

